Re: [Qemu-devel] [PATCH v20 4/7] virtio-balloon: VIRTIO_BALLOON_F_SG

[Tetsuo Handa]

Wei Wang wrote:
> >>>> @@ -173,8 +292,15 @@ static unsigned fill_balloon(struct 
> >>>> virtio_balloon *vb, size_t num)
> >>>>         while ((page = balloon_page_pop(&pages))) {
> >>>>           balloon_page_enqueue(&vb->vb_dev_info, page);
> >>>> +        if (use_sg) {
> >>>> +            if (xb_set_page(vb, page, &pfn_min, &pfn_max) < 0) {
> >>>> +                __free_page(page);
> >>>> +                continue;
> >>>> +            }
> >>>> +        } else {
> >>>> +            set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
> >>>> +        }
> >>> Is this the right behaviour?
> >> I don't think so. In the worst case, we can set no bit using 
> >> xb_set_page().
> >
> >>
> >>>                                If we can't record the page in the xb,
> >>> wouldn't we rather send it across as a single page?
> >>>
> >> I think that we need to be able to fallback to !use_sg path when OOM.
> >
> > I also have different thoughts:
> >
> > 1) For OOM, we have leak_balloon_sg_oom (oom has nothing to do with 
> > fill_balloon), which does not use xbitmap to record pages, thus no 
> > memory allocation.
> >
> > 2) If the memory is already under pressure, it is pointless to 
> > continue inflating memory to the host. We need to give thanks to the 
> > memory allocation failure reported by xbitmap, which gets us a chance 
> > to release the inflated pages that have been demonstrated to cause the 
> > memory pressure of the guest.
> >
> 
> Forgot to add my conclusion: I think the above behavior is correct.
> 

What is the desired behavior when hitting OOM path during inflate/deflate?
Once inflation started, the inflation logic is called again and again
until the balloon inflates to the requested size. Such situation will
continue wasting CPU resource between inflate-due-to-host's-request versus
deflate-due-to-guest's-OOM. It is pointless but cannot stop doing pointless
thing.

Also, as of Linux 4.15, only up to VIRTIO_BALLOON_ARRAY_PFNS_MAX pages (i.e.
1MB) are invisible from deflate request. That amount would be an acceptable
error. But your patch makes more pages being invisible, for pages allocated
by balloon_page_alloc() without holding balloon_lock are stored into a local
variable "LIST_HEAD(pages)" (which means that balloon_page_dequeue() with
balloon_lock held won't be able to find pages not yet queued by
balloon_page_enqueue()), doesn't it? What if all memory pages were held in
"LIST_HEAD(pages)" and balloon_page_dequeue() was called before
balloon_page_enqueue() is called?

So, I think we need to consider how to handle such situation.